The explosion of small portable electronic devices such as cell phones has led to the desire to be able to recharge the device without the necessity of attaching a cord to the device. A solution that accomplishes this task is known as “wireless power”. The term “wireless power” as utilized herein refers to the transmission of electrical energy from a power source to an electrical load without interconnecting wires. A common form for wireless power transmission utilizes two electromagnetically coupled coils to form a transformer through which power is transferred from the primary side to the receiving side. The transmitter may take the form of a pad having a coil embedded therein. The receiver may be built into a cellular telephone, for example, with the receiving side coil built into the back thereof. Although there is no direct contact between the transmitting and receiving coils, the close proximity of the coils and the judicious use of shielding allows for efficient transfer of energy from the transmitting side to the receiving side to operate a load, which may be a rechargeable battery being recharged by the system, for example.
FIG. 1 shows a block diagram of a prior art wireless power transmission system, generally as 100. The system comprises a transmitter side 102 and a receiver side 122. The transmitter side 102 comprises a circuit 104 for rectifying an AC input into a DC voltage which is fed into a power stage 106 for generating a high frequency signal. The high-frequency signal is coupled across a transformer 120 to the receiver side 122. The power stage 106 is controlled by controller 108 which could be combined into a single integrated circuit with the power stage 106. The receiver side 122 comprises a rectifier circuit 124 to output a DC voltage and a voltage conditioning circuit 126 which is operated by the receiver controller 128 to supply power to a load 130, which may be a rechargeable battery being recharged by the system, for example.
As shown FIG. 1, power flows from left to right from the transmitter to the receiver and communications flows from right to left from the receiver to the transmitter. The communication signals may be command signals to adjust the power level from the transmitter or other parameters, for example. The communication signals may be generated by coupling a resistor or capacitor across the receiving coil to generate signals which can be recognized by the controller on the transmitting side. The low-level signals are noisy because of the noise generated by the power transmission portion of the system.
The Wireless Power Consortium (WPC) defines a standard for such wireless power transmission. In a WPC defined wireless charging system, the power transmitter detects the signal from the power receiver as a modulation of current through and/or voltage across the primary circuit through a V/I circuit 110. In other words, the power receiver and the power transmitter use amplitude modulated power signals to provide a power receiver to power transmitter communication channel.
The WPC defined communication channel assumes that the incoming power signal is always amplitude modulated. However, that may not be a valid assumption. Accordingly, there is a need for a reliable, low-cost and easily integratable solution for detecting information being sent from the receiver to the transmitter in a wireless power system.